This invention relates generally to cutting monocrystalline semiconductor ingots into multiple wafers, and in particular to an apparatus and method for simultaneously slicing at least two semiconductor ingots to improve throughput.
Semiconductor wafers are generally prepared from a single crystal ingot, such as a silicon ingot, that is cylindrical in shape. The ingot is sliced in a direction normal to its longitudinal axis to produce as many as several hundred thin, disk-shaped wafers. The slicing operation may be accomplished by means of a wire saw, wherein the ingot is contacted with a reciprocating wire while a liquid slurry containing abrasive grains is supplied to a contact area between the ingot and the wire. As the abrasive particles in the slurry are rubbed by the wire against the ingot, silicon crystal is removed and the ingot is gradually sliced. The wire saw provides a gentle mechanical method for slicing which makes it ideal for cutting silicon crystal, which is brittle and could be damaged by other types of saws (e.g., conventional internal diameter saws). After slicing, each wafer is subjected to a number of processing operations to reduce the thickness, remove damage caused by the slicing operation, and create a flat and highly reflective surface suitable for installation of integrated circuit devices.
Wire saws generally have three or four rollers which are rotatably mounted on a frame, each roller having guide grooves for receiving segments of wire. Multiple parallel lengths of the wire extend between two of the rollers to form a wire web for slicing the ingot into multiple wafers. The space between adjacent wires in the web generally corresponds to the thickness of one wafer before processing. The apparatus includes an ingot support that may mount one silicon ingot and is adjustable to accurately align an orientation of the crystalline structure of the ingot relative to a cutting plane. The support is moveable in translation to bring the ingot into contact with the wire web.
Slurry is transported from a nearby slurry container to the wire by a pump, tubing, and at least one nozzle which dispenses slurry onto the wire web. A portion of the slurry then moves with the wire into a contact area between the wire and the ingot where the silicon crystal is cut. Typically, there are two nozzles positioned on opposite sides of the ingot holder so that slurry is dispensed onto the web on both sides of the ingot, thus facilitating delivery of slurry to the cutting region for either direction of travel of the reciprocal wire. Each nozzle is positioned above the wire web at close spacing and configured to dispense slurry in a generally thin, linear distribution pattern, forming a curtain or sheet of slurry. The slurry curtain extends across a full width of the wire web so that slurry is delivered to every reach of wire and every slice in the ingot.
A substantial concern when slicing semiconductor ingots is maintaining flatness of the wafers that are cut by the wire saw. One key to avoiding thickness variation and warp on wafer surfaces is controlling build up of frictional heat at the contact area, or cutting region. Accordingly, the liquid slurry is actively cooled prior to dispensing on the wire web so that it may remove heat as it passes through the cutting region. A heat exchanger is typically located between the container and the nozzle for cooling the slurry.
A limitation to the process of slicing semiconductor ingots is that it requires a substantial amount of time and can become a hindrance to the efficient production of wafers. It is desirable to slice the ingots as quickly as possible to improve throughput and reduce costs, yet there have been difficulties implementing a more rapid wire sawing process. The speed of the cutting wire cannot be substantially increased because that would elevate temperature at the cutting region to the detriment of the flatness of the wafers.
Thus, there is presently a need for improving the throughput of wire saws without compromising quality of the wafers cut.